A tilt rotor or tilt wing aircraft typically employs a pair of rotor systems which are supported at the outermost end of a wing structure and are pivotable such that the rotors may assume a vertical or horizontal orientation. In a horizontal orientation (i.e., horizontal rotor plane), the aircraft is capable of hovering flight, while in a vertical orientation (i.e., vertical rotor plane), the aircraft is propelled in the same manner as conventional propeller-driven fixed-wing aircraft.
Currently, tilt rotor and tilt wing aircraft employ conventional fixed-diameter rotor systems which are aerodynamically and aeroelastically designed in a manner that attempts to blend the competing requirements for hovering and forward flight modes of operation. For example, with regard to hovering flight, it is generally advantageous to employ a large diameter rotor to improve hovering performance by lowering disk loading, reducing noise levels, and reducing downwash velocities. Conversely, a relatively small diameter rotor is desirable in forward flight to improve propulsive efficiency by minimizing blade aero-elastic properties, minimizing blade area, and reducing tip speed (Mach number).
Variable Diameter Rotor (VDR) systems are known to provide distinct advantages over conventional fixed-diameter rotors insofar as such systems are capable of successfully operating in both modes of operation. That is, when the plane of the rotor is oriented horizontally, the rotor diameter is enlarged for improved hovering efficiency and, when oriented vertically, the rotor diameter is reduced for improved propulsive efficiency.
An example of one VDR system and VDR blade assembly is shown in U.S. Pat. No. 3,768,923 which discloses a blade assembly with an outer blade segment configured to telescope over a torque tube member. The size of the rotor diameter is varied by controlling the extension and/or retraction of the outer blade segment. The outer blade segment includes a structural spar which carries the primary loads of the outer blade segment, a leading edge sheath assembly and trailing edge pocket assembly, which sheath and pocket assemblies envelop the spar section to define the requisite aerodynamic blade contour. The torque tube member is mounted to the rotor hub assembly. The spar member of the outer blade segment slides over the torque tube member. In addition to supporting the outer blade segment, the torque tube member functions to transfer flapwise and edgewise bending loads to and from the rotor hub while imparting pitch motion to the outer blade segment.
A retraction/extension mechanism is located within the torque tube member and spar. In one embodiment of the invention, the retraction/extension mechanism includes a threaded jackscrew which may be driven in either direction by a bevel gear arrangement disposed internally of the rotor hub assembly. The jackscrew engages a plurality of stacked nuts which are permitted to translate axially along the jackscrew upon rotation thereof. Centrifugal load straps extend from each nut and are affixed via a retention plate to the tip end of the spar member. As the jackscrew turns, the stacked nuts are caused to translate inwardly or outwardly, thereby effecting axial translation of the outer blade segment. Systems relating to and/or further describing VDR systems are discussed in U.S. Pat. Nos. 3,884,594, 4,074,952, 4,007,997, 5,253,979, and 5,655,879.
U.S. Pat. No. 5,299,912 discloses another retraction/extension mechanism for a variable diameter rotor system. The retraction/extension mechanism includes coaxial rotor shafts which are engaged with the rotor blade. More particularly, the outer rotor shaft is attached to a rotor hub through a gimbaled bearing and provides rotational control over the rotor blade. The inner rotor shaft is attached to a bevel gear which, in turn, meshes with a bevel pinion mounted on a jackscrew. The jackscrew is engaged with the outer blade segment as described above. Rotation of the inner shaft produces corresponding rotation of the jackscrew, which extends or retracts the outer blade segment.
The primary drawback of the prior art systems is that they were designed primarily for developmental purposes. Those designs typically did not accommodate the dynamic loads that are produced in full size hardware designs, including gear tooth bending loads, and hertz stress to accommodate the torque load required to operate a driving screw member. The driving screw torque is a direct function of the centrifugal force created by the components of each blade assembly.
A need, therefore, exists for an improved gimbaled rotor head configuration for a variable diameter rotor system.